What does «si» mean in Spanish?

Implies or denotes condition or hypothesis. Example: If it rained I would go by car.
At the beginning of a clause, it emphasizes expressions of doubt, desire or assertion. Example: If you denied it yesterday, how dare you affirm it today?
Preceded by as or that is used in comparative concepts.
In indirect language, it is used to express the statement. Example: Tell me if you want to go to the movies.
In ponderative expressions, it is equivalent to how much. Example: ¡mira si sabe este niño!
It is used in exclamations of surprise. Example: ¡si será posible!
Seventh note of the scale.
Yes, chemical symbol for silicon.
(sí) Reflexive form of the third person pers. pronoun always used with a preposition. Example: de sí; por si; para sí.
(sí) To give of oneself, to lengthen, to stretch.
(sí) Of itself or of itself, by the very nature of the thing or person in question.
(sí) For himself, mentally.
(sí) To come to one's senses, to come to one's senses.
(sí) It is used to answer in the affirmative. Example: Do you have enough money? Yes.
(sí) Of course it is. Way to affirm emphatically.
(sí) Now that's just not, way to flatly deny.
(sí) Because yes, an affirmative answer to something without wanting to give the reason. Example: Why do you insist so much? Because I do.
(sí) Well, yes, we're all set. Example: there is no more money.
(sí) Well, yes...!
(sí) Consent. Example: saying yes.